What version of Guppy are you using? My guess is it's an old version that was released prior to your kit being released.
$ singularity exec --nv ONT_Guppy_GPU.sif guppy_basecaller --version
Guppy Basecalling Software, (C) Oxford Nanopore Technologies, Limited. Version 6.0.1+652ffd1
$ singularity exec --nv ONT_Guppy_GPU.sif guppy_basecaller --print_workflows
flowcell kit barcoding config_name model version
FLO-FLG111 SQK-RBK110-96 included dna_r10.3_450bps_hac 2021-04-20_dna_r10.3_minion_promethion_384_72309afc
As an aside, even with a small MinION run, 2 callers with 1 thread per caller will take a very long time to run. It would be worthwhile trying to get a GPU build and a supported GPU if at all possible. Run times go from dozens of hours to a few minutes from experience.
If your guppy build is not GPU friendly, then the parameter
-x cuda:all should be redundant since they refer to GPU cores.
Finally, since your kit is a barcoded multiplexed kit, I'm assuming you want to demultiplex the samples too. I have no experience with this, but it appears you are missing the demultiplexing parameter. Check out the ONT guide here.
EDIT: Just realised you said the error is it is not a GPU build. I thought that was a statement about it. You'll need a membership account with ONT to download and build a new version of the GPU caller. It looks like you have an old CPU only version.